This invention is in the field of integrated circuits, and is more specifically directed to voltage regulator circuits of the low dropout type.
As is fundamental in the art, voltage regulator circuits are commonly used circuits for generating a stable voltage from an input voltage supply that may vary over time, and over varying load conditions. Especially in automotive applications and in battery-powered systems, the demand is high for voltage regulators that can generate a low-noise stable output voltage with a minimum difference in potential between the input voltage and the regulated output voltage (the minimum potential difference is referred to as the "drop-out" voltage). Typical modern low drop-out (LDO) voltage regulators have drop-out voltages that are on the order of 200 mV.
Modern portable electronic systems, such as wireless telephones, portable computers, pagers, and the like also present additional requirements upon voltage regulator circuits. As known in the art, many modern integrated circuits are operating at increasingly lower power supply voltages, with 3.3 V power supply voltages now common in these systems, and with sub-1-V power supply voltages expected within the near future. These low power supply voltages are greatly desirable in portable electronic systems, because of their improved reliability, power efficiency, and battery longevity. Additionally, because voltage regulator circuits must remain operable at all times, the quiescent current drawn by these circuits is an important characteristic, as any reduction in this quiescent current translates directly into longer battery life. Finally, the fast switching times and high frequencies at which modem integrated circuits operate in turn require excellent frequency response on the part of the voltage regulator circuitry.
An example of a modem LDO voltage regulator is described in Rincon-Mora, et al., "A Low-Voltage, Low Quiescent Current, Low Drop-Out Regulator", Journal of Solid-State Circuits, Vol. 33, No. 1 (IEEE, January, 1998), pp. 36-44. As described therein, a current mirror circuit generates a significant boost current to assist an emitter follower at the output of the error amplifier, improving the slew-rate performance of the regulator while maintaining stability throughout the load-current range. In effect, the current mirror pushes the parasitic pole at the emitter of the emitter follower to a higher frequency during high load-current conditions, matching the increase in frequency of the required placement of this pole with increasing load current. Absent the current mirror and the resulting movement of the parasitic pole, more quiescent current flow than is necessary at low load current conditions would be required to ensure stability at high load currents. The current mirror ratio is preferably maintained relatively high to minimize power consumption.
By way of further background, copending application Ser. No. 08/992,706, filed Dec. 17, 1997, entitled "A Low Drop-Out Voltage Regulator With PMOS Pass Element", commonly assigned herewith and incorporated by reference hereinto, describes another LDO voltage regulator. In this regulator, a positive feedback path is provided from the current mirror to a source follower that is controlled by the output of the error amplifier; the positive feedback modulates the gate-to-source voltage of the source follower proportionally with the output device, to compensate the source follower for changes in the output impedance of the regulator. In this circuit described in this copending application, the positive feedback path includes an RC network to slow the response of the positive feedback relative to negative feedback provided to the error amplifier, in order to prevent oscillation of the circuit. Of course, this RC network reduces the bandwidth of the frequency response of the positive feedback.